A Wide Area Network (WAN) which employs an Internet Protocol-Virtual Private Network (IP-VPN) technique or the like is generally used as a communication network between bases, used for cloud computing or the like.
In a case where a terminal present in a certain base communicates with a terminal present in another base, the communication is performed via a line which connects a self-base LAN to a WAN and a line which connects the WAN to another base LAN. In these lines, an available bandwidth is restricted depending on contract bandwidths.
A TCP is generally used in communication between terminals. In the TCP communication, with respect to data which is transmitted by a transmission terminal, a reception terminal notifies of a position of the received data to the transmission terminal in a feedback manner. When the position of the received data of which the notification is sent in a feedback manner does not increase, the transmission terminal determines that discarding is detected.
In addition, the transmission terminal manages a parameter called a congestion window size (a size of data which can be transmitted even if a notification of reception thereof is not sent from the reception terminal) and varies the congestion window size depending on a round trip time (RTT) or whether or not discarding is detected.
It is determined that a network is congested when the RTT increases or discarding is detected, and a window size is reduced so as to indirectly reduce a transmission bandwidth, thereby preventing the network from being congested. In addition, it is determined that the network is vacant when the RTT decreases or there is no discarding, and the window size is increased so as to indirectly increase the transmission bandwidth, thereby effectively using a line bandwidth of the network.
As the RTT increases, the window size is hard to increase, and thus there are cases where bandwidths are unfairly assigned when communications having different RTT share a single line.
As above, the transmission bandwidth is greatly dependent on the RTT and a discarding rate in the communication using the TCP.
In addition to the RTT and the discarding detection, there is a technique of controlling a bandwidth through a combination of a transmission bandwidth in the past and a most recent retransmission bandwidth, or of controlling a bandwidth by using information of the number of connections exchanged between devices (PTL 1).